Fighting Fire in a Warmer World

Fires used to be nature’s way of keeping forests healthy. They would burn slowly through the hills and forests of California every decade or so, clearing out underbrush and making room for more plants to grow and animals to roam. Forest fires seldom claimed mature trees, which were sturdy and hydrated enough to handle the heat.

Not anymore. Due to a century of aggressive fire suppression and the escalating effects of climate change, fires have become more destructive and frequent. In 2015, a recordsetting 10 million acres in the United States were lost to fire, mostly in Alaska, California, and other western states.

Professor Andrew Latimer, second from the left, and his former Ph.D. student Jens Stevens, left, visit the Angora Fire site near Lake Tahoe with members of the U.S. Forest Service. The team traveled to 12 sites in California in 2013 to study the effects of fuel treatments and forest fire.

Experts say the future could be worse.

“There are 66 million dead trees in the Sierras because of drought and an alarming increase in bark-beetle infestation,” said Mark Schwartz, a professor with the Department of Environmental Science and Policy.

Catastrophic fires will change the landscape of California. Already, shrubs and grasslands are growing where forests used to stand. But UC Davis researchers are helping develop strategies to slow the transformation and give forests a chance to adapt to their warming world.

The forest-carbon connection

Forests are a carbon “sink.” They absorb more carbon dioxide than they release and reduce atmospheric greenhouse gases by about 25 percent globally. But when forests are destroyed by fire, they return that stored carbon to the atmosphere, which increases the effects of global warming and the likelihood of more frequent, intense fires.

“It’s a self-reinforcing loop,” said Ben Houlton, a professor in the Department of Land, Air and Water Resources.

Wildfires consume more than forestland. Fueled by drought and winds, fires rage nearly year-round through shrub-covered chaparral in California and beyond. Chaparral supports valuable ecosystems, as well as thousands of homes, but those landscapes don’t absorb carbon the way forests do. Also, chaparral regenerates more successfully than forests where many trees can’t reseed if fire destroys their cones.

How fire suppression backfired

Starting in the early 1900s, Americans became very good at fighting fires. Using engines, bulldozers, and aircraft, officials extinguished blazes before they spread. The result: dense forests.

“You had all these branches and needles piling up on the forest floor,” said Andrew Latimer, a professor in the Department of Plant Sciences. “Plus, young trees that would have been killed in small fires were able to grow. Forests have become more dense and filled with fuel.”

That fuel is even more combustible as temperatures rise and snowpack melts. After years of drought, trees are suffering from lack of water and falling victim to bark beetles, native pests that feed on weakened pine and other trees.

“A healthy tree can fight off bark beetles,” said Jens Stevens, a former Ph.D. student in Latimer’s lab who recently completed a postdoctoral position in the UC Davis John Muir Institute of the Environment. “Water-stressed trees are much more vulnerable.”

That adds up to millions of dead trees in forests filled with dry debris during fire seasons that stretch several months. No wonder California is experiencing more supersized fires.

What can be done?

UC Davis researchers work with the U.S. Forest Service and other fire officials to try and manage the extraordinary fire risk. One strategy is to “zone” the forest to maximize resources.

“There’s a huge backlog of forest that needs thinning and not enough resources to get to it all,” Latimer said. “The idea is to let remote areas burn more freely and focus on protecting homes and high-value groves like the giant sequoias.”

Researchers also study a vexing issue with regeneration. Not all, but many species of conifers need help reseeding themselves when wildfires scorch huge numbers of trees and the cones where they store their seeds. Latimer’s lab is testing how weather affects regeneration to determine if seedlings perform better in moist soil.

“If so, that could help the Forest Service prioritize when to reseed areas affected by wildfire,” Latimer said.

Sometimes, forest officials don’t have the time, money, or workforce to treat charred land. In 2013, the Forest Service didn’t have the budget to reseed the 250,000 acres the Rim Fire burned in the Stanislaus National Forest.

“Now shrubs and grassland are growing where the forest used to be,” said Schwartz, who predicts more changes to come.

“In the face of drought, disease and catastrophic fire, we’ll see more upslope migration of forest,” Schwartz said. “But forests will survive. Nature is resilient. Our goal is to help slow the pace of climate change and give ecosystems time to respond, realign, and adapt.”